Servo cylinder



April 7, 1953 w. SHEPPARD SERVO CYLINDER Filed Feb. 16 1951 f/M l HM Patented Apr. 7, 1953 SERV() CYLINDER William L. Sheppard, Romulus, Mich., assignor to Chrysler Corporation, Highland Park, Mich., a

corporation of Delaware Application February 16, 1951, Serial No. 211,300

(oi. coqin) `11i Claims.

v This invention relates to a uicl operated servo mechanism and is particularly concerned with control valving associated with the servo mechanism that is adapted to modify the action 0f .the servo. The invention is shown applied to a band type clutchor brake or similar device operated by a vservo mechanism rwherein the associated'valv ing provides an improved means for taking up slack in the band and also -provides anovel means for cushioning the application rof vthe brake or clutch band 'to its associated .drum element.

A particularly advantageous application of the invention 'herein disclosed may be made in :motor vehicle power transmission units that utilize ihye draulically operated servo mechanisms to provide means for controlling the transmission of differs ent speed ratio drives vthrough the power transe mission unit. While the `above mentioned specific use of the invention .disclosed Vis of primary con.-

cern, still, it is thought to be obvious lvthat lthere are numerous other applications and thus there is vno intention to limit l'this invention to the motorvehic'le power transmission art,

It is a primary object of this invention to provide a rfluidoperated servo mechanism that includes a combination slack take-up and force cushioning means :that modifies the action ofthe associated servo mechanism.

It is another yobject -of this invention to provide control valving for a fluid operated -servo mechanism that Awill automatically vinsure gradual application vof the rforce .exerted bythe servo mechanism.

AvIt is :aturther object fof this invention -to :pro-

vide control valving `for a fluid :operated `servo mechanism :that causes va more or less step-bystep increase in the application of the force exerted by the servo-mechanism.

Itis a still iur-ther object of this `invention to provide Aa iiuid operated servo mechanism having relatively movable piston 'elements mounted `there-- in fand .arranged lsuch that the force applied to :the Vpiston elements 'is transmitted in -a predetermined ma'nneiz Y A -It is -a still further object .of this invention to provide a iluid operated servo mechanism with 'contained valving yadapted to ,provide l.an .improved means -for taking up slack a .hand .device .operated by the servo mechanism.

Other .objects and .advantages :of the disclosed .invention `will-bereadil-y apparont'ifrom a reading of the yattached description :and a :considerationof .the related drawing wherein: l V

Fie. kl ,is a fragmentary sectional lelevaticurail view of a :bondrlvoe servo mechanism that embodies one form of this invention;

Fig. 21s an enlarged sectional view of the servo mechanism shown in Fig. 1, the view being taken along the line 2.-2 of Fig. l; Y

Fig. 3 is an enlarged sectional elevational view similar to Fig. 2 but showing the servo mechanism after pressurized fluid has been admitted to the piston receiving bore to effect application of the associated brake band; Y

Fig 4l is asectionalelevational View teken along the line 4.-4 of Fie 2: and

Fig. 5 is a sectional elevational View. Similar to Fig. 2, of a lmodiiieol form of ythe invention.

Fig.. l of the drawing shows ya rotatable bri-alte drum lll having ,an axially extending Peripheral flange Ill thatis adapted to be selectively engaged by abano-type brake element AI k2.. Themed-:type brake element :f2 preferably has vits inner .sur-isos covered .with a friction lining I3 that is normally spaced radially outwardly from the drum flange il.. Band'element .l2 substantially .enciroles the drum flange `Il and has an anchorelement VIl xed to the outer surface of oneend theroof- Anchor element i4 is formed to receive and abut against one end of yan adjustable anchor bolt 'VI- 5. Bolt lI5 is threadably mounted in a boss portion .ls of the relatively .stationary housing ll- All chor bolt l5 is provided with a :slotted head t8 :to provide for ready adjustment of the bolt l5 in the `boss I6, A lock mit I9 is threaded on the -holt l5 and arranged for engagement .with the outer .end of boss |-6 so vas to provide a means for-main taining the selected adjustment o f the :bolt l5.

The other end of the band .type brake element l2 is provided with yayolfze element 2| adapted to reeceive and support apivot pin 2 2. Pivot pinr22 pivotally connects the thrust link 23 to the yoke 2l that isxed .on the aforementioned other end .of the band 12. n

Housing Il is also provided with .an earv-likeprovjection 25 that journals .a pivot ypin 26. VPivot -pin 26 pivotallyconnectsa thrust lever 2.'| :to the housing all. lThrust `lever''l has ainotch 2,8 formed in an .edge thereof adapted :to seat .the free end Vor the thrust link 23.

Housing` I 'Iis also formedwith an integral pis.- 'ton receiving cylinder 3L It is obvious that the cylinder 3l could .be a .separable element that could be detachably connected to the housing H, Cylinder 3l includes the stepped ;bore32 that slidably receives the piston assembly .33, Pres- `surizecl fluid-is admitted to cylinder bore 11.32 from a Supply .Condut' through :an linlet port 36in brakeedevice operated :by 1a 355 lst-heheadorolosel:endiofeylinqergj lfiresSmfmed 21 clockwise will contract band I2 about drumV ange Il and provide a braking action onthe' drum I0. On release of the pressurized fluid in cylinder bore 32 the compression spring element 39, that is mounted in the lower open side of pistn bore 32, will move the piston assembly 33 upwardly to relieve the brake applying force. The inherent resilience of the band I2 will .expand the band I2 out of engagement with the drum flangell when the brake applying force is relieved. Positive acting compression spring means 4I may be placed between the opposed ends I4, 2l of the band element I2 to insure quick release of the band I2 from drum II when the fluid generated brake applying force is relieved. The-invention disclosed herein relates to the servo cylinder 3l and the piston assembly V33 reciprocably mounted in the cylinderSI. It has long been a problem as to how to effectively and economically take up the clearance or slack during the engagement of a brake mechanism of the type disclosed and at the same time provide a cushioned engagement of the band and drum that will prevent grab o1- jerkiness during brake engagement. If the full effect of the actuating pressure fluid in bore 32 is applied to the band actuating piston assembly 33, as in conventional servo mechanisms, it is obvious that'the piston 33 initially begins to move downwardly at a relatively -fast rate due to the initial movement merely taking up the slack and overcoming'the resistance of the spring 39. After taking up the slack or clearance between the band and drum, the full eiect of the pressurized applying iluid is suddenly applied Ito the rotating drum and as a re- -sult a grabbing and jerky engagement results. The degree of this rough engagement of the brake band will vary with the magnitude of the reaction vforces that must be taken by the brake band I2 and its supporting housing I1. If this brake band is utilized to control a planetary gear 'train of a motor vehicle power transmission unit then at times the gearing controlled by the band mayrequire the band to transmit a brake reaction of 2 or 3 to 1 and under such conditions the lgrabbing and jerkiness described above may be lquite objectionable to say the least. The several forms of servo mechanisms herein disclosed provide effective means for readily taking up the clearance between the brake band and brake drum and at the same time provide means for vobtaining a cushioned engaging action.

In the form of the invention shown particularly in Figs. 2 and 3, the head or closed end 40 of the cylinder 3| has a stud-like element 44 projecting axially inwardly from the inner sur- -face thereof. The stud elementV 44 may be of stepped formation having a base portion 44a of a relatively large size and a rib-like stem portion v44h Vof a relatively small size. Y Reciprocably mounted within the bore 32 of Icylinder 3l is the piston assembly 33. Piston assembly 33 comprises a pair of telescopically arranged, male and femalepiston elements 46 vand 41 respectively. Male piston element 4B is of a T-shaped cross sectional configuration having a head portion 5I and an integral stem portion 52. The peripheral edge of the head portion 5| is provided with a sealing ring 53 that slidably engages the side wall 32a of the cylinder bore 32. The'stem portion 52 of the piston element 46 is pierced by an axially rextending bore 54 that also extendsthrou'gh the head portion 5I so as to directly connect the cylinder bore 32 to the piston bore 54. The end of bore 54 adjacent the Vpiston head 5I is provided with an interiorly disposed, stepped, valve seat 55 that is adapted to be sealingly engaged by a ball valve 55. A spring 51, xedly mounted in the bore 54 by means of pin-58, continuously urges the ball valve 56 on the seat 55. When seated the ball valve 56 projects outwardly beyond the end of projection 55 by the amount e (see Fig. 3). It is thought to be obvious that the valve 56 provides a one-way valve .that permits pressurized uid to be transferred from bore 32 to bore 54 while preventing the flow of fluid in the opposite direction;

Telescopicallyv mounted about the stem portion 52 of the piston element 45 is the cupshaped piston element 41. The bore 6I in the piston element 41 slidably receives the stem 52 of piston 45 with a minimum of clearance therebetween. The headY or bottom 62 of the piston element 41 carries the nger 31 that engages the thrust lever 21 of the brake band actuating linkage previously described. The side wall 63 of the cup-shaped piston 41 is pierced by a pair of relatively small size ports or bores 64 and 55 respectively. The bores 64 and 65 are adjacent to but spaced axially from the piston head portion 62 at different distances as indicated at c and d in Fig. 3. The bores B4 and 65 are preferably of different diameters and in the construction shown the bore 64 is larger than the bore 65. The spacing of the bores 64 and 65 and their relative sizes has a material eiect on the operation of thisV servo mechanism as will hereinafter becomes quite evident.

Operation of the servo mechanism shownv in Figs. 1-3 isv as follows: When pressurized fluid is not being directed into the bore 32 of cylinder 3l then spring 39 will move the pist'on element v4I; to the released position shown in Figs. 1 and 2. The resilience of the band I2 andthe action of the band release spring 4I will seat the piston element 41 on the step 59 in the stem 52 of pis- Y ton element 45 when pressurized iluid is not being applied tothe cylinder bore 32. When spring 35 has moved the piston 46 to the released position shown in Fig. 2, then the outwardly projecting, axially extending, seat portion 60 on the head 5I of piston element 46 will engage and seat upon the stem portion 44h of the cylinder mounted stud 44. When the cylinder head stem 44b moves into engagement with the seat portion 5B of the piston element 46, stem portion 44b of Vthe cylinder head 40 also engages the ball valve 56 and depresses the ball 56 or its seat 55. Ball 55 is unseated on engagement of the piston portion 60 with the cylinder portion 44h due to the ball 5S normally projecting beyond the end of porjection 60 (see Fig. 3). rThus whenv the piston element 46 is seated on the cylinder head stud 44 there is an open passage or conduit connection v`between the cylinder bore 32 and the piston bore 54 that will permit the free now of uid between the cylinder bore 32 and the pistonbore 54. 1t will be' noted that on1y a slight amount or axial VmovementV of the piston 46 away from cylinder head 411 is required to seat the valvel 56. Rib 4417 of the stud 44 is arranged such that it engages both the ball valve 56v and the piston projection 60 when the piston 46 is nioved to its released position (see Fig. l) thus the slightest axial movement of piston 46 begins the seating of the valve 56. g 1 y When pressurized lluid is first admitted to the cylinder bore 32 to initiate application ofthe brake band l2 to the' drum l0, the pressurized u'id V'passes through the bore 32 and fills piston borel 54 due to the unseated ball valve 56. After filling bore 54 the pressurized fluid therein causes relativev expansion of the telescopically 'arranged piston elements'Y 45, 41 such that the piston element 41 moves downwardly away from the piston element'rll. There is an initial downward movement of the piston element 41, prior to any movement of the piston element 46 due to the fact that the relatively stiff spring 36 tends 'to Iiold the piston element 46 seated on the cylinder` stud 44. During this initial movement of piston 41 relative to piston 46, the valve 56 is unseated due to the piston 46 being held stationary against the rib 441). As the piston 'element 41 moves downwardly relative to the piston element l4,6, it initiates actuation of the brake band supplying linkage 21, 23 and takes up the slack or clearance between the band l2 and drum I6. After the band slack hasbeen taken up pressure fluid in the bore 54 and chamber 16 between piston elements 46 and 41 (see Fig. 3) begins to exert a force on theb'and I2 that then begins to apply the band l2 to the drum i6. It will be noted that during movement of the piston element 41 from the released position shown in Fig. 2 to the position shown in Fig. 3, wherein all slack or clearance between the band l2 and drum i6 has been taken up, 'that the bleed bores 64 and 65 in the wall 63 of piston 41 have both been uncovered. Uncovering bleed bores 64 and 65 permits the pressurized iuid trapped in the chamber and piston bore 54 to escape at 'a controlled rate and to return to the uid supply sump (not shown) Once the band slack" has been taken up* vby the downward moveinentoipiston 41, then the downward 'movement of the piston element 41 is retarded, due lto the increased reaction resulting from the beginning of application of the band i2 to the drumfl, and thereafter the piston element 46 will begin to Yinc'ive Off its seat 44o on the head of the cylinder v3l land telescope within the Ypiston element 41. Movement of piston element is orf its seat 44h will begin closing of the'v'alve Y56. As previously pointed vout only a very small axial movenient, distance e of Fig. 3, is required to seat the valve 56. After valve 56 seats then the pressure fluid trapped in piston bore AVA5&1 and chamber 1D has its pressure progressively increased with the succeeding telescopic compression of the piston `elements 46, 41. When valve 56 'rst closes both bleed bores 64 and 65 are uncovered 'due -to vthe previous expansion of piston element 41 relative to piston element 46. vAccordingly, the 'force transmitted to the piston element -41 by initial downward movement of the piston element '4"6 is cushioned somewhat by the escape vfrom bleed bores 64 and 65 of the pressure uid'tr'apped in chamber 10. With lsubsequent continued 'movement of the piston element 46 downwardly Yrelative tothe element v41, the relatively large bleed bore 64 is closed off and thereafter the pressure of 'the fluid trapped in bore '54 and 'chamber 'in materiaiiy increasesso that theband essere l2 isapplied to drum l0 with an increased force. Continued movement of the piston'element 46l downwardly relative to the' piston element 41 will increase the pressure of the iluid trapped in chamber 16 and Will subsequently close off the relatively small bleed bore 65. Thereafter the pistons 46 and 41 actas substantially a single rigid unit and the full eiect of the pressure fluid in chamber 32 acting on the head 52- 'of the piston 46 is directly applied to the brake band actuating linkage. What little pressurized fluid that is trapped in the chamber 10 after closing 'of bleed bore 65 eventually drains back to the uid supply sump (not shown) through the clearance 6l between the vslidably engaged sides of the telescopic pistons 46 and 41.

From the above description it is thought to be obvious that there are about four `successive steps in the application of the force exerted by the servo mechanism on the associated brake band I2. First there is the slack or clearance take-up step when the valve 56 is open and pressure fluid admitted to bore 54 causes the pistons d6, 41 to expand relative to one another. Next there is the step when the piston 46 moves oi its seat 4412 and closes valve 56 to begin the telescopic contraction of the piston elements 46 and 41. At lthis time both bleed bores 64 and 65 are effective to 'control vthe force Vbuild-up applied to lever 21. Next there is the step when large bleed bore 64 has been closed off and small bleed bore 65 alone is effective to control the force build-'up in the servo mechanism. Finally there -is the concluding step when both bleed bores 64 and 65 have been closed 01T and the full effect of the pressure iluid in cylinder bore 32 is Veifective to apply the brake band l2. While the four pressure variation eects have been described as definite steps for the sake of clarity, it is to be realized that the band applying action occurs very rapidly and that there is a blending or graduation of one step into the next step so that a very smooth continuous slack take-up and band engaging action results. By control of the spacing and size of the bleed bores 64, 65 it is possible to vary the operating characteristic of the servo mechanism to meet certain predetermined conditions. The operation of the servo mechanism described has been predicated on the `fact that a more or less limited supply of pressure fluid is directed into cylinder bore 32. This supply may be limited by a restriction 15 in the pressure fluid supply line 36. 1

Fig. 5 shows a modified form of the invention that is quite similar to the form shown in Figs. '1 4. The Fig. 5 form differs from the Fig. 1 4 form only in the construction of the inner surface '4ta of the head 411 of cylinder 3l and in the construction of the head end 5i' of the piston element 46. All other elements of the two forms of the invention are identical and like reference numerals have been assigned to identical elements. The exposed outer side 8l of the head ei of piston 46 in the Fig. 5 construction is Aformed with an annular groove 82 that receives "a resilient sealing ring 83. Ring 83 may be of rubber or some similar material. Head El' is also provided with an annular, axially extending ange 84 that abuts the outer peripheral side ofthe groove 82 landthe resilient ring 63. Flange '84 supports the ring V33 and prevents its expan- A number of 7,. 82 to the sump or drain side of the servo mechanism. Bleed bores 85 relieve the pressure on the inner end edge of the sealing ring 33 and tend to retain the ring 83 in the groove 82. With the Fig. form of the invention, it is obvious that when the piston 4S is in its released position, as shown, that only the area of piston head 5i that is encircled by the ring 83, is acted on by the pressure fluid admitted to the cylinder bore 32. Thus ring t3 provides a means for controlling the force applied to the piston i6 whenthe piston 46 is in its released position, This means for controlling the force on the piston i6 in its released position is important in that it provides a means for varying the slack take-up operating characteristics of the servo mechanism with changes in diameter of the ring S3. Obviously after piston 4e starts to move away from the seat 44 then the seal between ring 83 and cylinder head surface ita is broken and then the entire area of the head 5i of'piston it is acted on by the pressurized fluid admitted to cylinder bore 32 to bring about the aforementioned cushioned engagement of the brake band l2 with the drum I0.

I claim: l. A pressure uid operated servo mechanism comprising a cylinder with a bore therein, a pair of relatively movable pistons reciprocably mounted in said cylinder bore, a pressure duid inlet to said cylinder bore arranged to introduce pressurized servo energizing fluid to said cylinder bore to successively effect relative and conjoint movement of said pistons, valve means carried by one of said pistons and controlled by the positioning of said one of said pistons in one of its limiting positions in said cylinder bore to control the application of said pressurized servo energizing fluid to the other of said pistons to eiect relative movement between said pistons, and pressure ud bleed porting in one of said pistons and controlled by the relative movement between said pistons to bleed the pressurized servo energizing fluid from said servo mechanism to provide means to cushion the force transmitted by the energization of the pistons of said servo mechanism.

2. A pressure uid operated servo mechanism comprising a cylinder with a bore therein, a pair of relatively movable pistons reciprocably mounted in said cylinder bore, a pressure duid inlet to said cylinder bore arranged to introduce pressurized servo energizing fluid to said cylinder bore to effect relative and conjoint movement of said pistons, valve means operated by the positioning or" at least one of said pistons in one of its limiting positions in said cylinder bore to control the application of said pressurized servo energizing iluid to the other of said pistons to effect relativeimovement between said pistons, pressure fluid bleed porting connected with said pistons and controlledV by the relative movement between said pistons to bleed the pressurized servo energizing iluid from said servo mechanism to provide means to cushion the force transmitted by the actuation of the pistons of said servo mechanism, and resilient means arranged to resist movenient of said pistons by pressurized fluid introduced to said cylinder bore. Y

3. A servo mechanism comprising a cylinder with a bore therein, a pair of telescopically arranged pistons reciprocably mounted in said cylinder bore, a pressure iiuid inlet to said cylinder bore arranged to apply pressurized servo energizing fluid to at least one of said pistons to successively eiect relative and conjoint movement of said pistons, a valve controlledY conduit means carried by one of said piston means and operated by movement of said one of said pistons to and from its limiting positions adapted to provide for controlled transfer of pressurized servo energizing fluid from said cylinder bore to a chamber formed interior-ly of said telescopically arranged pistons from opposed portions of said pistons, admission of pressurized servo energizing fluid to said chamber being eiective to cause relative Vmovement between sai-d pistons, pressure fluid bleed porting associated with interengaged, relatively movable portions of said pistons t0 bleed pressurized servo energizing fluid from said chamber so as to cushion the effect of the force transmitted by the pistons of said servo mechanism, and resilient means resisting conjoint movement of said pistons.

4. A servo mechanism comprising a cylinder with a bore therein, a pair of telescopically arranged pistons reciprocably mounted in said cylinder bore, a pressure fluid inlet to said cylinder bore arranged to apply pressurized uid to at least one of said pistons to eiect conjoint movement thereof, a valve controlled conduit means carried by one of said piston means adapted to provide for controlled transfer of pressurized iiuid from said cylinder bore to a chamber formed interiorly of said telescopically arranged pistons from opposed portions of said pistons, admission of pressurized iiuid to said chamber being effective to cause relative movement between said pistons, a plurality or" pressure fluid bleed ports in the walls of said chamber to provide for controlled escape of the pressurized fluid introduced to said chamber, said bleed ports being spaced apart axially of said telescopically arranged pistons and being of varying sizes, and resilient means resisting conjoint and relative movement of said pistons.

5. A servo mechanism comprising a cylinder with a bore therein, a pair of telescopically arranged large and small pistons reciprocably mounted in said cylinder bore, a pressure fluid inlet to said cylinder bore arranged to apply pressurized servo energizing fluid to said pistons to successively enect relative and conjoint movement or the pistons, a valve controlled conduit means carried by the larger of said pistons and operated by movement of the larger of said pistons to and from one of its limiting positions adapted to provide for controlled transfer of pressurized servo energizing uid from said cylinder bore to a chamber formed interiorly of said telescopically arranged pistons from opposed portions of said pistons, admission of pressurized servo energizing fluid to said chamber being effective to cause relative movement between said pistons, pressure fluid bleed porting formed in interengaged, relatively movable portions of said Vpistonsto bleed pressurized fluid from said chamber on relative movement between the pistons so as to cushion the eiect of the force transmitted by the pistons of said servo mechanism, resilient means resisting conjoint and relative movement of said pistons, and mechanism engageable with and actuable by the smaller piston of said servo mechanism. Y

6. A pressure fluid operated servo mechanism comprising a cylinder with a bore therein, a pair of pistons reciprocably mounted in said cylinder bore for conjoint and relative movement therein, a pressure iiuid inlet to said cylinder bore arranged to apply pressurized servo energizing 19 fluid to one lof `said ,pistons to urge said one piston in one direction, resilient means resisting movement of said one piston by the 'pressurized fluid admitted to said cylinder bore, conduit means arranged to :apply the pressurized servo energizing fluid in said cylinder bore to a cham- `Lblar formed by opposed portions of said pistons to effect movement of the other piston in said one direction when said Aone piston is in one of its limiting positions, valves means operated by the movement of lsaid one piston from said limiting position in said one ldirection to `prevent the passage of the pressurized servo energizing fluid lfrom the cylinder bore through said conduit means to the said chamber, pressure fluid bleed porting in the other of said pistons controlled by the relative movement between said pistons to cushion the effect of the pressure fluid introduced to said cylinder bore and to said chamber, and means resisting movement of said other piston in said one direction.

7. A pressure fluid operated servo mechanism comprising a cylinder with a bore therein, a first piston of substantially T-shaped cross sectional conflguration having the head portion thereof reciprocably mounted in said cylinder bore and the stem portion extending axially of the cylinder bore, a pressure fluid inlet to said cylinder bore arrangedv to apply pressurized fluid to the head portion of said first piston to effect movement of said first piston in one direction in said cylinder bore, resilient means resisting movement of said first piston in said one direction, a second piston of substantially U-shaped cross sectional configuration having the side wall portions thereof telescopically mounted on the stem portion of said first piston and arranged to provide a pressure fluid receiving chamber of variable capacity between the free end of the stem portion of the first piston and the closed end of the second piston, a bore through the stem and head of said first piston connecting the pressure fluid receiving portion of the cylinder bore to said chamber, a valve mounted in said bore to control passage of pressurized iiuid from said cylinder bore to said chamber, abutment means in said cylinder bore to open said valve when said cylinder bore is free of pressurized fluid, resilient means continuously urging said valve closed, and resilient means opposing movement of said second piston in said one direction.

8. A pressure fluid operated servo mechanism comprising a cylinder with a bore therein, a first piston of substantially T-shaped cross sectional configuration having the head portion thereof reciprocably mounted in said cylinder bore and the stem portion extending axially of the cylinder bore, a pressure fluid inlet to said cylinder bore arranged to apply pressurized fluid to the head portion of said first piston to effect movement of said first piston in one direction in said cylinder bore, resilient means resisting movement of said first piston in said one direction, a second piston of substantially U-shaped cross sectional configuration having the side wall portions thereof telescopically mounted on the stem portion of said first piston and arranged to provide a pressure fluid receiving chamber of variable capacity between the free end of the stem portion of the first piston and the closed end of the second piston, a bore through the stem and head of said rstpiston connecting the pressure fluid receivinglportion of the cylinder bore to said chamber, a valve mounted in said bore to control passage'offpressurized fluid from said cylinder here .to said chamber, abutment means in Said cylinder bore to open said valve when said cylinder ,boire is free of pressurized fluid, resilient means continuously urging said valve closed, pressure fluid bleed porting piercing the side walls of said second piston and connectible to said chamber to provide va means to bleed pressurized fluid from said chamber during certain relative positions fof said pistons, and resilient means opposing movement of said second piston in said one direction.

9. A pressure fiuid operated servo mechanism comprising a cylinder with a bore therein, a first piston of substantiallyv T-shaped cross sectional configuration having the head portion thereof reciprocably vmounted in said cylinder bore and the stem portion extending axially of the cylinder bore, a pressure fluid inlet to said cylinder bore arranged to apply pressurized `fluid to the head portion of .said first piston to effect movement of said first piston in one direction in said cylinder bore, resilient means resisting movement of said first piston in said one direction, a second piston of substantially U-shaped cross sectional configuration having the side wall portions thereof telescopically mounted on the stem portion of said first piston and arranged to provide a pressure fluid receiving chamber of variable capacity between the free end of the stem portion of the first piston and the closed end of the second piston, a bore through the stem and head of said first piston connecting the pressure fluid receiving portion of the cylinder bore to said chamber, a valve mounted in said bore to control passage of pressurized fluid from said cylinder bore to said chamber, labutment means in said cylinder bore to open said valve when said cylinder bore is free of pressurized fiuid, resilient means continuously urging said valve closed, a plurality of pressure fluid bleed ports in the side wall portion of said second piston, said bleed ports being spaced apart axially of the second piston and being of varying sizes with the size of the bores increasing from the closed end of the second piston towards the open end thereof, said bleed bores being connectible to said chamber at predetermined relative positions of said pistons, and resilient means opposing movement of said second piston in said one direction.

10. A servo mechanism of the type described in claim 9 wherein the effective area of the head portion of the first piston is equal to the area of the cylinder bore and the effective area of the vclosed end of the second piston is considerably smaller than the area of the cylinder bore.

1l. A servo mechanism of the type described in claim 9 wherein the head portion of the first piston includes an axially extending, annular projection arranged concentrically'within the cylinder bore, said projection being adapted to seal against a wall of the `cylinder when the first piston is in a released position to provide a reduced area to which is initially applied the piston yactuating pressurized fluid.

12. A servo mechanism of the type described in claim 9 wherein the entire area of the head portion of the first piston is always effective to transmit the force of the pressurized fluid admitted to the cylinder bore.

13. A servo mechanism adapted to apply a band type brake element to a rotatable drum member comprising a cylinder with a closed end bore, a pressure fluid inlet through the closed end of the cylinder, a first piston including a head portion reciprocably mounted in said bore, said head porextending bore piercing said stem and head portions of said rst piston, a Valve member resiliently mounted in said bore having portions normally projecting axially outwardly of said head portion of the first piston and adapted to be engaged with the closed end of the cylinder bore, a second piston telescopically mounted on the free end of the stem portion of the rst piston, said second piston having ya closed head portion adjacent the free end of the stem portion of the first piston and side wall portions surrounding said stem portion, said side Wall portions being pierced by openings of different sizes that are spaced axially from the closed end of said second piston at 4variable intervals, and resilient means resisting movement ofthe pistons by admission of pressure uid to the cylinder bore.

Y WILLIAM L. SHEPPARD.

f2 REFERENCES CITED The following references are of record inthe le of this patent:

UNITED 'STATES PATENTS Number Name Date 1,970,999 Ferris et al Aug. 2l, 1934 2,170,851 Carroll Aug. 29, 1939 2,453,785 Cousno Nov. .16, 1948 2,513,192 McFarland June 27 1950 FOREIGN PATENTS A Number Y Country Date 521,980 Great Britain June 5, 1940 

